Confirmation bias is the natural human tendency to selectively search out and believe data that confirms one's preconceptions. This type of cognitive bias can lead a person to ignore data that is inconsistent with or contradicts those preconceptions. Numerous studies have shown that confirmation bias is a major cause of false prosecutions and erroneous convictions.

Anchoring bias is another type of cognitive bias that can contribute to or reinforce confirmation bias. For example, during a price negotiation session, the first number voiced may become the anchoring point around which the negotiation proceeds. Similarly, the first witness a police officer interviews may lead the officer to believe that a certain person or a member of a certain group committed the crime in question.

One may think that confirmation bias and anchoring bias occur only when an investigator is considering subjective or qualitative data. But actually, these types of cognitive biases can affect the perceptions of forensic evidence analysts.

"Nor is it evident that what respondent calls 'neutral scientific testing' is as neutral or as reliable as respondent suggests. Forensic evidence is not uniquely immune from the risk of manipulation." - Justice Antonin Scalia, Melendez-Diaz v. Mass., 129 S.Ct. 2527 (2009)

Given the fact that people routinely and naturally rely on confirmation bias and anchoring bias to guide their actions, what are some important facts a criminal defense lawyer must keep in mind when defending a client?

Keith A Findlay, Associate Professor at the University of Wisconsin Law School makes these conclusions in his presentation Cognitive Bias in Forensic Science:

• Forensic evidence is not always infallible.

• Top-down analysis by experts (which relies on pre-existing knowledge), is particularly prone to manipulation when the item of study is ambiguous.

• Confirmation bias leads people to seek confirming evidence rather than disconfirming evidence.

• Investigators tend to rate exonerating evidence as less reliable than guilt-confirming evidence that supports their initial impressions.

• Investigators tend to show marked confirmation bias when asked to state a hypothesis of guilt early in an investigation than when asked toward the end of an investigation.